Within the semiconductor industry, projection printers including reticles (also called masks) are used to pattern resist layers. A reticle typically is a glass plate with chrome members on the plate. The chrome members are opaque to a radiation wave having a specific wavelength. The reticle areas without chrome members are substantially transparent to the radiation wave. A discussion of projection printing and diffraction limitations of projection printing appears on pages 274-76 of VLSI Technology edited by S. M. Sze (.COPYRGT. 1983), which is herein incorporated by reference and hereinafter referred to as Sze.
Within this application, special terminology is used. "Radiation pattern" is being defined as the pattern of radiation formed at the surface of a resist layer when a radiation wave passes through a reticle. Dimensions as used in the application refer to the length and width of a portion of the radiation pattern at the resist layer surface or a portion of the patterned resist layer. Both dimensions are within a plane substantially parallel to the substrate surface (x-y plane) as seen during an aerial view. Thickness is in a vertical direction substantially normal to the substrate surface (z-axis) as seen during a cross-sectional view. Having defined the terminology, the discussion of the prior art continues.
Resolution is a linear measure of how small a radiation pattern dimension can be resolved. Depth of focus is a linear z-axis measure over which a radiation pattern remains in focus. Formulas for calculating resolution and depth of focus appear on page 276 of Sze. When the dimensions of a patterned resist layer are larger than the resolution and the resist layer thickness is thinner than the depth of focus, a substantially ideal resist layer profile as shown in FIG. 1A is formed. The profile includes resist members 10 and 11 that are about the same thickness as the resist layer as originally coated over substrate 19. There is substantially no resist within each opening 13. The comers of the profile are substantially square.
Many prior art methods use a radiation pattern to form the resist layer pattern, such that the pattern and dimensions of both the radiation pattern and the resist layer are substantially the same. When the dimensions are less than the resolution, the patterned resist layer formed by prior art methods is not substantially ideal because of diffraction as disclosed in Japanese Patent Application Number 63-295350 by Okamoto, which is herein incorporated by reference and hereinafter referred to as Okamoto. The light intensity incident of the radiation pattern at the resist layer is similar to the intensity profile that appears in FIG. 18(d) of Okamoto. A resist layer profile similar to the one shown in FIG. 1B of this application is formed by the intensity profile of Okamoto's FIG. 18(d). The resist layer profile has regions 14 that are substantially the same thickness of the resist layer as coated over substrate 19. Regions 15 are not substantially exposed and have some resist material in locations that are supposed to be openings. Therefore, the regions 15 are not openings and may cause problems during subsequent processing steps. Region 16 receives a significant amount of light and forms a region where the resist thickness is significantly thinner compared to the regions 14. The region 16 may cause problems during subsequent processing steps because the region 16 is expected to have a thickness similar to regions 14. The comers formed within the profile are rounded and are generally not desired.
As can be seen by the resolution and depth of focus equations in Sze, both are affected by changing a wavelength of the radiation wave or the numerical aperture of a lens within the projection printer. Therefore, smaller resolution can be achieved by decreasing the wavelength or increasing the numerical aperture, but the smaller resolution is accompanied by a shallower depth of focus. The shallower depth of focus may require a thinner resist layer and is not generally not desired because of processing complications during subsequent processing steps caused by the thinner resist layer. U.S. Pat. No. 4,869,999 includes a disclosure of a patterning method that uses a plurality of exposures along the optical axis (z-axis) as opposed to the substrate surface (x-y plane).